I have two cloned repositories of two very similar open-source projects, which I have been working on in different instances in Sublime Text 2 to arrive at my desired result. Co
There are a number of diff plugins available via Package Control. I've used Sublimerge Pro, which worked well enough, but it's a commercial product (with an unlimited trial period) and closed-source, so you can't tweak it if you want to change something, or just look at its internals. FileDiffs is quite popular, judging by the number of installs, so you might want to try that one out.